WASHINGTON, D.C. — On a 30-mile stretch of railroad between Westerly and Cranston, R.I., Amtrak’s 150-mile-per-hour Acela hits its top speed — for five or 10 minutes. On the crowded New York to Washington corridor, the Acela averages only 80 m.p.h., and plans to bring it up to Japanese bullet-train speeds will take $150 billion and 26 years, if it ever happens.